1. Field of the Invention
The invention relates to correlation tracking of a spread spectrum coded signal, particularly with respect to tracking the clear/acquisition (C/A) modulation in the Global Positioning System (GPS).
2. Description of the Prior Art
The GPS is a navigation system utilizing a plurality of satellites in diverse orbital positions. Each satellite transmits an L-band carrir bi-phase modulated by a spectrum spreading Pseudo Random Noise (PRN) C/A code identifying the satellite. The C/A code is utilized by a GPS receiver to acquire and lock to the satellite transmission. In the present day system, the carrier frequency is 1575.42 MHz an the C/A code has a clock rate of 1.023 MHz and a code length of 1023 bits. The code repeats the predefined sequence thereof once each millisecond.
In order to receive the GPS signal structure, a GPS receiver generates an exact duplicate of the C/A code and aligns it in time to the receive code using correlation detection. The correlation detection apparatus of the GPS receiver advances and retards the locally generated C/A code until a correlation peak is detected. Following code acquisition, the correlation detection apparatus tracks the code correlation peak. The corrlation detection apparatus utilizes a phase shifter to advance and retard the phase of the code clock by fractional bit phase shift increments to achieve acquisition and tracking. The accuracy and resolution of GPS receiver correlation detection apparatus in the acquisition and tracking functions thereof are limited by the phase shift increment provided by the phase shifter. The phase shift increment is dependent upon the available clock rate. Typical GPS receives utilize a 0.1 microsecond phase shift increment available from a 10.23 MHz clock rate.
It is desirable in present day GPS receiver designs to provide better resolution. In the prior art, this is achieved by utilizing a higher frequency code clock and/or a more complex phase shifting device to obtain the higher resolution. The prior art approach adds significant expense to the receiver because of the cost of the higher frequency hardware. This is particularly disadvantageous in the design of a low cost GPS receiver.